The invention relates to a steering device for a motor vehicle and a method of controlling a steering device.
Modern steering devices of a motor vehicle are nowadays typically assisted electronically, so that the steering torque applied by the driver of the motor vehicle is amplified by an electronic steering assist unit. In this way, the driver will perceive the process of steering the motor vehicle to be generally more comfortable. Also, the steering assist unit can generate a counter-torque so that especially at higher speeds, an increased steering torque needs to be applied by the driver for steering. This is beneficial to operational safety.
The electronic steering assist unit typically includes a transmission and an electric motor, which result in a certain friction and mass inertia of the steering device. In particular, the automatic, and also desired, return behavior of the steering device, with which the driver is familiar from steering devices that are not electronically assisted, is adversely influenced. The adverse influence is such that although the wheels do return when transitioning from a cornering to traveling straight due to the track of the vehicle, the driver will not perceive a torque associated with the return of the wheels at the steering element.
In the prior art, the return behavior is actively provided by the electronic steering assist unit by an appropriate assist torque being generated for the return motion. To this end, the electronic steering assist unit includes an open- and closed-loop control unit which detects the steering angle, the steering wheel angular velocity and the vehicle speed by means of sensors and drives the electric motor accordingly, so that, by way of the assist torque generated, the driver is given the impression that the steering element returns automatically.
However, it has turned out to be a disadvantage here that influencing factors for the respective vehicle topology still have to be taken into consideration and compensated in a time-consuming manner since the steering angle and the steering wheel angular velocity are detected at the steering shaft, which, depending on the vehicle type, have different transmission ratios in relation to the wheels. Further influencing factors include, for example, the steering axis inclination, the caster and the steering offset of the steering axle geometry.